In conventional brushless motors, use has been made of a method of detecting a magnetic pole position with omission of a magnetic-pole-position detecting sensor which is constituted of a resolver or a Hall-effect device.
When the rotation of a rotor is allowed at the time of starting in reference to a method of the sort mentioned above for detecting a magnetic pole position, it is possible to utilize a technique, as disclosed in Japanese Patent Publication No. 3-239186A, for determining a rotor position (the magnetic pole position) by switching between a synchronous operation mode wherein steady operation is performed and a rotor-position detecting mode wherein the magnetic pole position is detected, rotating the rotor by supplying such a gate pulse as to generate a rotating magnetic field in each three-phase armature coil at the time of starting a brushless motor and then causing a position detecting circuit to detect the voltage induced in the armature coil.
When the rotation of the rotor is not allowed at the time of starting, that is, when the magnetic pole position is estimated with the motor unoperated, there is a technique, as proposed in IEEJ IAS Vol. 116-D, No. 7, pp. 736-742 (1996) for determining the magnetic pole position with the motor unoperated by supplying intermittent pulse-like voltage commands sequentially in a given direction to the extent that the motor is unrotated, and estimating a position angle from a difference of response, which varies in an anti-sinusoidal wavelike manner, of each of the phase currents i.alpha..sub.u and i.beta..sub.u, i.alpha..sub.v and i.beta..sub.v, i.alpha..sub.w and i.beta..sub.w, etc., which are converted into the static coordinates.
However, there still exist the following problems in the aforesaid prior art. The technique disclosed in Japanese Patent Publication No. 3-239186 is not applicable to a motor which makes it a condition that its rotor is at a standstill before the operation of the motor because the motor has to be started by rotating the rotor in order to determine the rotor position.
In the case of the technique made known by the IEEJ IAS Vol. 116-D, No. 7, electrical parameters such as the inductance of the brushless motor, resistance values or the like are needed to obtain the difference of the current response by deriving each of the phase currents i.alpha..sub.u, i.alpha..sub.v, i.alpha..sub.w or the like from a three-phase voltage equation. Consequently, the degree of difference of the current response is unclear in case these parameters remain unknown and since the voltage command is not a stepwise alternating command, overcurrent may flow, depending on the form of the voltage command, or the rotor may be rotated; thus, there arise problems in view of its practical use.
It is therefore an object of the present invention to provide a method of estimating the initial magnetic pole position of a permanent magnet type brushless motor adapted so that even though electrical parameters are not accurately acquired, the initial magnetic pole position of a rotor in the brushless motor can be estimated quickly without allowing overcurrent to flow and without rotating the rotor, namely, without operating the motor.